1. Field of the Invention
This invention relates generally to electro-optical imaging systems, and more particularly to a system and method for improving contrast in a reflective display projection system.
2. Description of the Background Art
Contrast is an important measure of an imaging systems performance. To match the performance of cathode ray tubes (CRTs) and/or film, contrasts of greater than 300:1 are required. Further, such contrasts must be achieved at low f-numbers to insure sufficient light throughput. In multi-colored systems, the contrasts levels must be achieved for each color.
FIG. 1 shows a typical reflective display based projection system 100, that illustrates the operation of a polarizing electro-optical imaging system. Projection system 100 includes an illumination source 102, a polarizing beam splitter 104, a color separator 106, a plurality of reflective liquid crystal displays (LCDs) 108(r, g, and b), and projection optics 110. Illumination source 102 generates a source beam of white light and directs the source beam toward polarizing beam splitter 104, which passes one portion of the source beam having a first polarity, and redirects another portion (an illumination beam) of the source beam having a second polarity along a system axis 112, toward color separator 106. Color separator 106 separates the illumination beam into its red, green, and blue components, and directs each of these colored illumination beams to a respective one of LCDs 108(r, g, and b). Each of LCDs 108(r, g, and b) is controlled by a system, e.g., a computer or other video signal source (not shown), and modulates the polarity of selective portions (i.e., pixels) of the colored illumination beams to form colored imaging beams, which are reflected back toward color separator 106. Color separator 106 recombines the colored imaging beams to form a composite imaging beam and directs the composite imaging beam back along system axis 112, toward polarizing beam splitter 104, which passes only the modulated portions of the composite imaging beam to projection optics 110. Projection optics 110 then focuses the modulated portions of the composite imaging beam onto a display surface (not shown).
The limiting contrast of an imaging system is defined as the maximum light output during an xe2x80x9cONxe2x80x9d state (Im) divide by the light output (leakage) during an xe2x80x9cOFFxe2x80x9d state (Il).
Limiting Contrast=(Im)/(Il)xe2x80x83xe2x80x83(Eq. 1)
Polarization aberrations reduce contrast in polarizing optical imaging systems, by introducing unwanted polarization changes in the light. These changes increase leakage (Il) in the case of an xe2x80x9cOFFxe2x80x9d state, thereby significantly reducing the limiting contrast.
One source of polarization aberrations is the field angle dependence of the polarization vector. In particular, when using a polarizing beam splitter, the orientation of the polarization vector is perpendicular to the plane of incidence of the light passing therethrough. In other words, the polarization of a light ray passing through the system depends on the particular angle that the ray makes with the polarizer. The greater the field angle, the greater the polarization aberration. This aberration is further increased as the f-number of the system is reduced to provide adequate light throughput. In systems such as system 100, the field angle dependence of polarizing beam splitter 104 can be compensated for by positioning a xc2xc-wave plate 114(r, g, b) in front of each LCD 108(r, g, b), respectively.
Another source of polarization aberrations is the angular variation in the polarization characteristics of the liquid crystal layers of imagers 108(r, g, b). LCD compensation films such as wide view polarizing films (manufactured and supplied by Fuji Optical Films and marketed by companies such as Nitto Denko and Polatechno) are used to reduce such aberrations in certain transmissive displays (e.g., lap-top computer displays). However, such compensation films cannot be used in reflective systems like system 100, because as the light makes a second pass (after reflection by the LCD) through the film, the compensation effect of the film is reversed.
What is needed, therefore, is a system and method for improving the contrast in an electro-optical imaging system (e.g., an LCD projector) by reducing polarization aberrations. What is also needed is a system and method for improving contrast in systems with relatively small f-numbers.
The present invention overcomes the problems associated with the prior art by providing an off-axis electro-optical imaging system (e.g., an LCD projector), wherein the illumination beam and imaging beam are separated to facilitate the use of a single-pass aberration compensation element. The invention achieves improved contrast at low f-numbers, thereby providing an important advantage over the prior art.
The projection system includes a polarizer, a reflective light modulator, an analyzer, and an aberration compensation element. The polarizer polarizes an illumination beam to form a polarized illumination beam. The reflective light modulator receives the polarized illumination beam along a first optical path, modulates the polarized illumination beam to form an imaging beam, and reflects the imaging beam along a second optical path not parallel to the first optical path. The aberration compensation element is disposed in at least one of the polarized illumination beam and the imaging beam. The separation of the illumination beam and the imaging beam provided by the off-axis architecture facilitates the use single-pass aberration compensation elements.
In one particular embodiment, the polarizer and the analyzer are sheet polarizers, oriented with their transmission axes perpendicular to one another. In a more particular embodiment, the aberration compensation element is a half-wave retarder. In an even more particular embodiment, the aberration compensation element is a sheet retarder oriented with its fast axis perpendicular to the nominal transmission axis of either the polarizer or the analyzer, to compensate for the field angle dependence of the transmission axes of the polarizer and the analyzer.
In another particular embodiment, the reflective light modulator is a liquid crystal display, and the aberration compensation element is a liquid crystal compensation film. The liquid crystal compensation film can be disposed in either the illumination beam or the imaging beam. Optionally, the liquid crystal compensation film is used in conjunction with a half-wave retarder. The liquid crystal compensation film and the half-wave retarder can be embodied in a single element, or can be spaced apart from one another.
A multi-channel embodiment is also disclosed. The multi-channel projection system includes a color separator, a plurality of polarizers, a plurality of reflective light modulators, a plurality of analyzers, a plurality of aberration compensation elements, and a color combiner. The color separator separates a multi-colored illumination beam into a plurality of colored illumination beams. Each of the polarizers polarizes a respective one of the colored illumination beams to form a polarized, colored illumination beam. Each of the reflective light modulators modulates a respective one of the polarized, colored illumination beams to form a colored imaging beam, and reflects the colored imaging beam in a direction not perpendicular to the surface of the display. Each analyzer analyzes a respective one of the colored imaging beams, and the color combiner combines the analyzed imaging beams to form a multi-colored imaging beam.